A drilling jar is a tool added to the bottom hole assembly (BHA) with an objective to provide powerful downward and/or upward impact during a stuck pipe incident. Its main application is to free a stuck pipe in a well. The need to maximize the efficiency of drilling jars drives the oil and gas industry to develop and improve drilling jar mechanisms in order to be able to deliver an efficient system and jarring up and/or down for a longer period of time. However, doubts about the tool's integrity as well as the performance and efficiency of the drilling jar remain. There are mainly three types of drilling jars: mechanical, hydraulic and hydro-mechanical drilling jars.
A mechanical drilling jar has a mechanism that is set to release at a specific preset load. To fire the drilling jar, a load (pull or push load) must be applied. If the load is not applied, the drilling jar may not fire. The drilling jar fires immediately once this predetermined load is reached. A mechanical drilling jar uses a mechanical latch mechanism, which is set to be released at a predetermined force, which is preset before running the drilling jar in the hole.
The mechanical drilling jar remains locked in position until a force exceeding the latch presetting load is applied to the drilling jar. This prevents the drilling jar from firing under normal drilling, tripping or handling operations. The mechanical drilling jar can be operated in either tension or in compression. However, there are two disadvantages with a mechanical drilling jar. The first disadvantage is that when the over pull/slack-off load exceeds the latch setting load, the drilling jar fires immediately. This increase in load could be intentional or unintentional. The resulting shock wave transmitted to the surface can damage the drilling rig hoisting equipment. The second disadvantage is that the drilling jar may fire immediately as there is no time delay. The two types of mechanical drilling jars are 1) with preset load latch setting, and 2) with a latch load adjustable downhole.
A hydraulic drilling jar overcomes the disadvantages of mechanical drilling jars. Its release mechanism is hydraulic. After the over pull is applied, there is a time delay, for example of about 30 to 180 seconds, before the drilling jar actually fires and delivers a blow. Tensile load applied to a hydraulic drilling jar creates internal pressure in the drilling jar. This pressure forces the hydraulic oil to move from one section to another (named chamber) in a controlled manner. This flow causes the delay in functioning of the drilling jar. The delay period is approximately 30 seconds to 3 minutes before the drilling jar is released. The firing load is variable by the amount of over pull or slack off on the drilling jar. The higher the over pull or slack off, the shorter may be the delay period.
Hydraulic drilling jars, however, do not have an internal latch, therefore they can easily transition from tension to compression. For this reason a safety clamp is required on the mandrel to hold the drilling jar in the open position, while racked back on derrick, before running in the hole. The safety clamp must support the weight of at least two drill collars above the drilling jar. Some advantages of hydraulic drilling jars include the time delay allows the driller to adjust the amount of over pull applied, and by doing this, the intensity of the jarring impact can be controlled. The time delay allows the driller to engage the drum brake before the drilling jar fires. This prevents the resulting shock wave from damaging the drilling rig hoisting equipment. Additionally, the minimum load to fire the drilling jar is just above the internal drag in the jar. Any load above the internal drag may cause the jar to fire contrary to a mechanical drilling jar, which requires exceeding the preset load. Normally, hydraulic drilling jars have a hydraulic mechanism to fire up, but a mechanical mechanism to fire down. Double Acting Hydraulic drilling jars (DAHDJ) are also available. These drilling jars have hydraulic firing mechanisms, in both the up and the down directions. Hydraulic drilling jars, however, have some limitations and restrictive operating procedures.
For example, the drilling jar must be kept in an extended (open) or compressed (closed) position at all times to prevent unexpected firing during normal drilling operations. Additionally, when running in the hole with the drilling jar extended, special care must be taken to avoid closing or “cocking” the drilling jar when passing through tight spots, ledges, or doglegs. Cocking is the term used to bring the drilling jar back in to working (jarring) position to fire again (up or down). If there is a possibility that the drilling jar may have closed, the string must be held in the elevators until the drilling jar fires into the open position. If the drilling jar fires while the drill string is supported in the slips, the resulting shock wave can cause the slips to jump out of the rotary table, releasing the drill string. If DAHDJ are run in compression (closed position), these may “cock” when picked up off bottom. It means that the drilling jar must be fired down lightly (the drilling jar allowed to bleed down) when the drilling bit tags bottom to minimize the down-jarring impact on the bit.
Additionally, when lifting off-bottom with the drilling jar in compression (closed position), the drill string must be picked up and held in the elevators, until the drilling jar has fired into the open position. Additionally, when back reaming during a tight hole situation, the wall drag can often support the BHA below the drilling jar. When the drill string is lowered to set the slips, the drilling jar may close to the set position. Picking up on the drill string again, may then cause the drilling jar to fire unexpectedly. When handling the drilling jar on the rig floor, a safety clamp must be installed to keep the drilling jar from closing into the “cocked” position. The safety clamp must not be removed until the drilling jar is in tension in the drill string. There is, however, a risk of the safety clamp being overlooked when running in the hole, preventing the drilling jar from functioning downhole.
The third type of drilling jar, which is most commonly used during drilling operations, is the hydro-mechanical drilling jar. It combines the hydraulic and mechanical releasing mechanisms. The hydro-mechanical drilling jar has the features of both hydraulic and mechanical drilling jars without the disadvantages of either type. This tool provides several distinct advantages over conventional hydraulic or mechanical drilling jars. For example, hydraulic time delay allows the driller to vary the over pull applied, and then apply the draw works drum brake. By doing this, the jarring force is controlled and damage to the drilling rig hoisting equipment is prevented. Location in BHA is less restricted. The jar can be used in tension or compression within the limits of the latch setting. No safety clamps or special handling procedures are required on the rig floor. The drilling jar may not fire unexpectedly when drilling, or tripping in or out of the hole. The latch mechanism prevents drilling jar movement during normal drilling operations, eliminating unnecessary wear of internal components. The latch resets automatically, locking the drilling jar in the neutral position. The linear action latch is not affected by torque, and there is double acting, i.e. jarring action in both up and down directions.